Posttraumatic stress disorder (PTSD) is a debilitating psychiatric disorder associated with high rates of chronicity, poor quality of life, and severe impairments in interpersonal, occupational, and social functioning. While evidence-based psychotherapies for treatment of PTSD have been developed and disseminated within the VA system, no treatment has shown universal effectiveness and there have been great concerns about attenuated treatment response and elevated treatment drop out in veteran populations. The evaluation of alternative treatment modalities for veterans with PTSD is therefore an important priority. The Minneapolis VA Health Care System (MVAHCS) has taught Mindfulness Based Stress Reduction (MBSR), a group-based intervention focused on mindfulness meditation, to veteran clinical populations since 2001, and pilot data from veterans diagnosed with PTSD is promising. Despite support for the application of MBSR to other mental health and physical problems, MBSR has not been systematically evaluated as a treatment for PTSD. Moreover, the existing literature on MBSR is limited by methodological weaknesses. If shown to be efficacious through scientifically sound trials, MSBR may offer an effective, acceptable, and tolerable intervention for veterans suffering PTSD who are unable to engage in or complete traditional exposure-based therapies. The primary goal of this proposal is to conduct a methodologically rigorous randomized controlled trial (RCT) of MBSR in treating PTSD among veterans, examining both symptom outcomes and subsequent health services utilization. Given our compelling pilot data, we propose initial steps to evaluate putative mechanisms of change (self-report and electrophysiology markers, i.e., EEG) through which MBSR may relate to PTSD symptom improvements, and to examine the acceptability of MBSR to the veteran population. The MVAHCS and the assembled team combines expertise in PTSD treatment and research, clinical trials, and neuroscience, with clinical expertise in MBSR and compelling pilot data to support the feasibility and scope of the current project. The efficacy of MBSR will be examined relative to present-centered group therapy (PCGT), a non-specific therapeutic comparison group. Veterans diagnosed with PTSD will be randomized to MBSR or PCGT for 9 weeks. Each intervention will be delivered in group format following manualization by trained clinicians receiving expert supervision. Treatment integrity will be independently monitored. Assessment of clinical outcomes post-treatment and 2 months follow-up will be independently evaluated. Putative mechanisms of mindfulness meditation will be assessed using self-report and electrophysiology markers. We have the following aims: Primary Aim 1: To evaluate the efficacy of MBSR as a treatment for PTSD in veterans compared to PCGT over 9-weeks of treatment and 2-month follow-up. Secondary Aim 1: To identify potential treatment-based predictors of PTSD outcomes. Secondary Aim 2: To identify differences in subsequent VA health services utilization among veterans across treatment conditions. Exploratory Aims: 1. To evaluate rates of drop-out, compliance, and consumer satisfaction with MBSR. 2. To evaluate acceptability and outcomes of treatment with veterans with mild traumatic brain injuries.